Of the new class of high temperature superconductors (i.e., transition temperature greater than 90.degree. K.), those consisting of an oxide of yttrium, barium and copper (YBa.sub.2 Cu.sub.3 O.sub.7-x) have received the most attention. While this material possesses a number of benefits, it also displays a number of shortcomings. In particular, careful control of the formulation and processing steps is necessary in order to form the orthorhombic structure of YBa.sub.2 Cu.sub.3 O.sub.7-x which exhibits superconductivity. Any deviations can lead to the formation of secondary phases which are not superconducting and which reduce the amount of superconducting material present in a given specimen. This material has exhibited poor resistance to atmospheric degradation caused by reaction of contained barium species with CO.sub.2 and H.sub.2 O naturally occurring in the air around us.
Incorporating fluorine into the YBa.sub.2 Cu.sub.3 O.sub.7-x structure will increase the percent Meissner effect, which is defined as the ratio of the Meissner magnetization to the diamagnetic magnetization at 4.5.degree. K., in direct relationship to the fluorine content after incorporation. This ratio increase indicates the presence of an increased amount of superconducting material in the treated structure or enhanced shielding.
Attempts to synthesize pure YBa.sub.2 Cu O.sub.7-x F.sub.y phases have included several different approaches. Solid state synthesis using YF.sub.3, YOF, BaF.sub.2, CuF.sub.2, NH.sub.4 HF.sub.2, or KHF.sub.4 as the fluorine source results in the formation of BaF.sub.2 with no indication of any phases with superconducting transition temperatures higher than pure YBa.sub.2 Cu.sub.3 O.sub.7. Ion irradiation with fluorine ions and fluorine (NF.sub.3) plasma reactions are processes for fluorine incorporation into the YBa.sub.2 Cu.sub.3 O.sub.7-x material. Gas phase exchange reactions have focused on F.sub.2 as fluorine source and show evidence of fluorine incorporation in samples prepared by treating YBa.sub.2 Cu.sub.3 O.sub.7-x. These products were either semiconducting, or if superconducting, did not show any improved properties over the pure 123 material. Evidence of fluorine substitution was also reported in samples of YBa.sub.2 Cu.sub.3 O.sub.7-x which were heated with ZnF.sub.2 ; however, this product was not superconducting.
Additional information concerning the state of the art can be obtained from the following references:
A. "Latest Results on High Temperature Superconductivity," S. R. Ovshinsky and R. Young, Energy Conversion Devices, presented at Drexel University Conference on Superconductivity, July 29-30, 1987. (Oral presentation with slides, no hard copy available).
B. "Chemical, Structural, and Electrical Property Studies on the Fluorination of the 90K Superconductor YBa.sub.2 Cu.sub.3 O.sub.6.8," H. H. Wang, et al. (Argonne National Lab), Communication to Inorganic Chemistry, Vol 27, No. 1, pp. 5-8, 1988.
C. "Fluorination of Superconducting Ba.sub.2 YCu.sub.3 O.sub.7-.delta., P. K. Davies, et al. (Univ. of Pennsylvania) Solid State Communications, Vol. 64, No. 12, pp. 1441-1444, 1987.
D. "Effects of Fluorination on the 90K Superconductor Ba.sub.2 RCu.sub.3 O.sub.7-.delta. (R=Y, Eu; 0&lt;=.delta.&lt;=1)," S. M. Fine, et al. (AT&T) to appear in Solid State Ionics as part of the Proceedings of the Reactivity of Solids Conference.
E. "Fluorine Oxidation of High-Temperature Superconductors," K. M. Cirillo, et al. (Univ. of Wisconsin) Solid State Communications, Vol. 66, No. 12, pp. 1237-1241, 1988.
F. "Fluorination of the High-T.sub.c Superconductors YBa.sub.2 Cu.sub.3 O.sub.7-.delta.," N. N. Sauer, et al. (Los Alamos National Laboratory) Journal of Materials Research, Vol. 3, No. 5, pp. 813-818, September/October 1988.
G. "Neutron Powder Diffraction Study of the Products from Fluorinating YBa.sub.2 Cu.sub.3 O.sub.7-.delta. at Room Temperature and at 400.degree. C.," E. Garcia, et al. (Los Alamos National Laboratory) Journal of Materials Research, Vol. 3, No. 5, pp. 819-824, September/October 1988.
H. "Conversion of La.sub.2 CuO.sub.4-.delta. to a Superconductor by Treatment with Fluorine Gas," B. M. Tissue, et al. (Univ. of Wisconsin) Solid State Communications, Vol. 65, No. 1, pp. 51-54, 1988.
I. "Fluorination of High T.sub.c Superconductors YBa.sub.2 Cu.sub.3 O.sub.x : Influence on the Superconducting Properties," C. Perrin, et al. (Universite de Rennes-I) Physica C 153-155 (1988) pp. 934-935.
J. "Passivation Process of High T.sub.c Superconductors YBa.sub.2 Cu.sub.3 O.sub.7-x by Fluorine Gas Treatment," A. Tressaud, et al. (Universite de Bordeaux) Modern Physics Letters B, Vol. 2, No. 10 (1988) 1183-1188.
K. "ESR Spectra of High T.sub.c Superconducting Oxides Treated Under Various Atmospheres," J. M. Dance, et al. (Universite de Bordeaux) Solid State Ionics (1988).
L. "Influence of Low-Temperature Fluorination Process on the Characteristics of Superconductors YBa.sub.2 Cu.sub.3 O.sub.7-x and Bi.sub.4 (Ca,Sr).sub.6 Cu.sub.4 O.sub.16+x," A. Tressaud, et al. (Universite de Bordeaux), Solid State Ionics 32/33 (1988).
M. "Fluorine-Implanted Bismuth Oxide Superconductors," R. P. Gupta, et al. (Central Electronics Engineering Research Institute) Applied Physics Letters, Vol. 54, No. 6, Feb. 6, 1989.
N. "Actual Fluorination of Y-Ba-Cu-O High T.sub.c Superconductors by a Solid-Gas Reaction Under NF.sub.3 Flow," C. Perrin, et al. (Universite de Rennes I) Superconductor Science and Technology (1989).